It's not possible to ship more than one parcel with the same label. Keep in mind that the label is like the ID of each parcel due to its barcode. The carrier organises collections and deliveries according to this data, so if every parcel is sent with the same label, there will only be proof of collection, transit, or delivery for one of them.
If the shipment purchased has serveral parcels, don't worry: the PDF file containing the shipment label will have one for each parcel, each one numbered (for example 1/3, 2/3, and 3/3).
If you use the same label for the same shipment, for all purposes, the carrier will only have knowledge of one parcel.